The manufacturing of semiconductor devices and related thin film circuitry usually involves etching of specific layers comprising the device. Typically, the area to be etched is masked by a material such as photoresist with the mask forming a pattern of lines and areas exposing the layer to be etched. In earlier approaches, the etching was carried out by a wet chemical method in which the etchant material, typically oxidizing mineral acids, contacted the exposed surface.
More recent processes for etching aluminum metalliziations employed gas plasmas, particularly chlorine-based gases selected from the saturated halocarbon series, which eliminate some of the undesirable effects of wet chemicals. However, in both methods the etching was basically isotropic. Additionally, conventional plasmas were incapable of quantitatively removing copper residues after etching a binary or ternary alloy of aluminum containing copper. With isotropic etching, random etching proceeds at a uniform rate in all directions. As the surface to be etched is removed, the etching action takes place not only vertically into the surface, but also horizontally against the edge of the recess created by the etching. Thus, the area to be etched suffers undercutting in which the material is etched not only vertically in line with the edge of the opening in the photoresist mask, but also it extends underneath the photoresist mask. Typically, this undercutting extends horizontally to substantially the same degree as the vertical etching.
As the trend toward miniaturization continues, scaling down to micron and submicron dimensions becomes necessary. This imposes strict demands on etch profiles, with insignificant undercutting. One mode of plasma etching, commonly referred to as reactive ion etching (RIE), produces anisotropic etching as a result of directional effects that suppress isotropic etching. Reactive ion etching combines physical momentum transfer cascade interactions (sputter etching) accomplished by energetic ions, along with chemical reactions normally observed in conventional plasma etching carried out by highly reactive neutral atoms or free radicals. The energetic ions are accelerated across the plasma dark space adjacent to the wafer in a direction perpendicular to the wafer's surface. The relatively low pressures employed during reactive ion etching, in the range of a few microns Hg to 100 microns Hg, substantially eliminate scattering collisions before the ion's impingement vertically on the wafer's surface. This ensures that the sputter etching component of RIE is anisotropic, thereby providing for a vertical etch wall on a plane closely approximating that delineated by the resist edge prior to the etching operation.
As techniques of lithography improve, line patterns of micron and submicron dimensions in photoresist images become possible. In order to effectively etch these patterns on the various substrates, reliable and reproducible anisotropic etching is necessary. Wet chemistry methods cannot effectively etch at these dimensions. With the demand for much smaller dimensions of line width and spaces, the lack of control and reproducibility resulting from undercutting, has made isotropic etching unacceptable.
Present dry plasma chemical etching and reactive ion etching techniques employ conventional chlorine-containing plasmas. While these plasmas, containing boron trichloride/chlorine binary mixtures, can etch anisotropically, they usually leave a residue of nonvolatile silicon and copper-containing compounds when applied to the patterning of aluminum-copper or aluminum-silicon-copper films. These nonvolatiles are highly hygroscopic, a condition that leads to severe and rapid post-etch corrosion of the patterned metal. This copper-rich residue may also be electrically conductive, potentially leading to short-circuited devices.
Another problem associated with very large scale integrated silicon circuits (VLSI) is the desirability that there be a reasonably high selectivity in the etch rate of aluminum metallizations with respect to the etch rate of underlying silicon, silicon oxides, or photoresist etch masks, to provide enough line dimensional control during the etching for the preservation of both the metallized interconnects and the underlayers.
It is therefore a primary object of the present invention to provide a plasma reactive ion etching technique for utilization during semiconductor fabrication in which highly efficient aluminum metallizations etching takes place with a high degree of selectivity, cleanliness, and dimensional control, wherein highly anisotropic etching takes place to provide substantially vertical profiles in the features etched in the metallized layer, where the removal process is primarily due to physical and chemical interactions ensuring complete removal of nonvolatile materials (e.g., copper) from the etched areas, thereby eliminating the requirement for subsequent wet-chemical treatment.